This invention relates to a nitride semiconductor device used for light emitting devices such as a light emitting diode (LED) and a laser diode (LD), light receiving devices such as a solar cell and a light sensor and electronic devices such as a transistor and a power device, especially relates to an improved quantum well structure light emitting diode having an emitting peak wave length in a range of 450 to 540 nm wherein a loared operating voltage and an increased output can be obtained.
Nitride semiconductors have been used to make high bright and pure green and blue LEDs for full color displays, traffic signals and light sources for image scanner and so on. These LED devices are made of laminated structures which basically comprise a GaN buffer layer formed on a sapphire substrate, a n-type GaN contact layer doped with Si, an single-quantum-well (SQW) or multi-quantum-well (MQW) active layer comprising InGaN, a p-type AlGaN clad layer doped with Mg and a p-type GaN contact layer doped with Mg. The SQW blue laser having a peak wave length of 470 nm has shown a very superior characteristic such as the output of 2.5 mW and the external quantum efficiency of 5% at 20 mA, whereas the MQW has shown the output of 5 mW and the external quantum efficiency of 9.1% at 20 mW. Further, the SQW blue. laser having a peak wave length of 520 nm has shown the output of 2.2 mW and the external quantum efficiency of 4.3% at 20 mA, whereas the MQW has shown the output of 3 mW and the external quantum efficiency of 6.3% at 20 mW.
The MQW are expected to get an improved device characteristic such as higher outputs as compared to the SQW because the MQW can emit the light efficiently at a small current due to a plurality of mini-band structures. As a typical LED device having the MQW active layer for getting a good efficiency and output, Japanese Patent Kokai Hei 10-135514 discloses a nitride semiconductor light emitting device which comprises a MQW light emitting layer comprising laminated structures of undoped GaN barrier layers and undoped InGaN quantum well layers between clad layers having a wider band gap than that of the barrier layer. However, in order to improve the output of the blue green LED having a longer peak wavelength, there is proposed the increased number of layers in the MQW structure. The forward voltage Vf becomes higher depending on the layer number of MQW, resulting in such problems as the higher forward voltage Vf and the lowered emitting output.
The object of the present invention is to provide a nitride semiconductor device having an active layer of a quantum well structure with large number of layers and relatively low forward voltage, especially with an improved emitting efficiency and a higher emitting output.
As a result of focused research for luminous phenomenon in the light emitting diode having a Multi Quantum Well (MQW) structure between n-type semiconductor layers and p-type semiconductor layers, there was found that recombination of electrons and holes in the MQW active layer mainly happen in a quantum well layer or layers close or proximate to the p-type nitride semiconductor layers and rarely happen in a quantum well layer close or proximate to the n-type nitride semiconductor layers. That is, the quantum well layer near to the n-type nitride semiconductor layers can hardly function as the emitting layer. After that, there was found that, when an n-type impurity is doped into the quantum well layer close to the n-type nitride semiconductor layers, the carrier density thereof increases, so that the forward voltage can be lowered and the emitting efficiency can be improved. The present invention was completed on the basis of the above finding. According to a first aspect of the present invention, there is provided a nitride semiconductor device which comprises an active layer containing an n-type impurity and comprising a quantum well layer or layers and a barrier layer or layers between n-type nitride semiconductor layers and p-type nitride semiconductor layers, wherein at least one of said barrier layers and said quantum well layers at the proximate side in said active layer to said n-type nitride semiconductor layers is dope with n-type impurity. In this invention, the donor can be additionally supplied into the active layer from the n-type nitride semiconductor layers, so that a higher output can be obtained. In a preferred embodiment of the present invention, the layers to be doped with the n-type impurity should be determined according to the following formula (1). Because, if the number of the doped layers are beyond the determined number, a good output can not be obtained due to the deterioration of crystal quality. If said active layer is a MQW having (i) laminated layers, then at least one of 1st to j-th layers counting from the side proximate to said n-type nitride semiconductor layers is doped with n-type impurity, wherein jxe2x80x2=i/6 +2, where i is an integer of at least 4, and wherein j is the integer portion of jxe2x80x2.
In the present invention, the layer doped with the n-type impurity means the layer intentionally doped with the n-type impurity, preferably in a range of 5xc3x971016 to 2xc3x971018/cm3. In a case that the layer contains the n-type impurity in a range of 5xc3x971016 to 2xc3x971018/cm3 due to the diffusion of the n-type impurity from the neighboring layer and the contamination from original materials and CVD devices, the unintentional doping layer also belongs to the doped layer.
Generally, the barrier layer and/or the quantum well layer is preferably an undoped layer for functioning as the emitting layer. In the present invention, the undoped layer means a layer not containing the n-type impurity of more than 5xc3x971016/cm3.
In a preferred embodiment of the present nitride semiconductor device, the barrier layer and/or the quantum well layer at the distal side to said n-type semiconductor layers may be not doped with n-type impurity. Therefore, in the preferred nitride semiconductor device having the active layer of a SQW, the quantum well layer and the barrier layer at the proximate side to said p-type semiconductor layers are not doped with n-type impurity. On the other hand, in a case of MQW the proximate layers to said n-type semiconductor layers may be doped with n-type impurity whereas said proximate layers to said p-type semiconductor layers may not be doped with n-type impurity.
In a preferred case, said active layer comprises 9 to 15 layers, at most 4, preferably at most 3 layers of which from said n-type semiconductor layers are doped with n-type impurity.
The above structure may be applied to the active layer comprises InxGa1xe2x88x92xN (0 less than x less than 1) suited to the emitting light of 450 to 540 nm, preferably 490 to 510 nm.
In a preferred embodiment, the n-type impurity may be selected from the group consisting of Si, Ge and Sn. The n-type impurity content of the active layer may be lower than that of said n-type semiconductor layers. In the other case, the n-type impurity content of the active layer may decrease depending on distance from said n-type semiconductor layers. Generally, the n-type impurity content of the active layer may be in a range of 5xc3x971016 to 2xc3x971018/cm3. Preferably the n-type impurity content of the barrier layer and/or the quantum well layer in the active layer may be in a range of 5xc3x971016 to 2xc3x971018/cm3.
In a typical case, the n-type impurity content of the barrier layer is in a range of 5xc3x971016 to 2xc3x971018/cm3, whereas the n-type impurity content of the quantum well layer is in a range of 5xc3x971016 to 2xc3x971018/cm3 and lower than that of the barrier layer. In another typical case, the n-type impurity content of the barrier layer is in a range of 5xc3x971016 to 2xc3x971018/cm3, whereas the n-type impurity content of the quantum well layer in the active layer is less than 5xc3x971016 to 2xc3x971018/cm3 and lower than that of said barrier layer.
In the present invention, for the improvement of higher output, the thickness of the barrier layer or quantum well layer close or proximate to said n-type semiconductor layers is larger than that of said barrier layer or quantum well layer close or proximate to said p-type semiconductor layers. For the improvement of low operational voltage, the thickness of the barrier layer or quantum well layer close or proximate to the n-type semiconductor layers is smaller than that of the barrier layer or quantum well layer close or proximate to the p-type semiconductor layers.
The inventive MQW structure can be preferably applied to a blue-green light emitting diode. Therefore, according to a second aspect of the present invention, there can be provided a nitride semiconductor emitting device which comprises an active layer comprising quantum well layer or layers and barrier layer or layers between n-type nitride semiconductor layers and p-type nitride semiconductor layers, wherein the quantum layer in the active layer comprises InxGa1xe2x88x92xN (0 less than x less than 1) having a peak wavelength of 450 to 540 nm and the active layer comprises laminating layers of 9 to 13, in which at most 3 layers from the side of the n-type nitride semiconductor layers are doped with an n-type impurity selected from the group consisting of Si, Ge and Sn at a range of 5xc3x97106 to 2xc3x971018/cm3.
In a typical embodiment of the present invention, the thickness of the barrier layer or quantum well layer close or proximate to the n-type semiconductor layers is larger than that of the barrier layer or quantum well layer close or proximate to the p-type semiconductor layers. In another typical embodiment of the present invention, the thickness of the barrier layer or quantum well layer close or proximate to the n-type semiconductor layers is smaller than that of the barrier layer or quantum well layer close or proximate to the p-type semiconductor layers.
In a preferred embodiment, the inventive MQW active layer can be preferably applied to the light emitting diode having the quantum well layer of InxGa1xe2x88x92xN (0 less than x less than 1) having a peak wavelength of 490 to 510 nm. In this case, the barrier layer may comprise InyGa1xe2x88x92yN (0xe2x89xa6y less than 1, y less than x).
In a more preferred embodiment, the active layer comprising an MQW of InxGa1xe2x88x92xN (0 less than x less than 1)/InyGa1xe2x88x92yN (0xe2x89xa6y less than 1, y less than x) lamination may be formed on an n-type multi-layer, which may be selected from the group consisting of a buffer super lattice layer undoped with n-type impurity and comprising InzGa1xe2x88x92zN (0 less than z less than 1)/GaN lamination or AlwGa1xe2x88x92wN (0 less than w less than 1)/ GaN lamination. In this case, the GaN layer of the buffer super lattice layer may have a thickness of less than 70 xc3x85 whereas the barrier layer of the active layer may have a thickness of more than 70 xc3x85.
In another preferred embodiment, the multi-layer may be doped with n-impurity and comprises lamination of GaN layer and a layer selected from the group consisting of InzGa1xe2x88x92zN (0 less than z less than 1, z less than y) layer having a larger band gap energy than that of the quantum well layer and AlwGawN (0 less than w less than 1) layer. In this case, the n-type impurity for doping into the active layer and the n-type clad layer is preferably Si and the Si content of the active layer may be in a range of 5xc3x971016 to 2xc3x971018/cm3 whereas the Si content of said n-clad layer may be in a range of more than 5xc3x971017/cm3 and larger than that of the active layer.